1. Field of the Invention
The present invention relates to a control circuit and a control method for a current mode control type DC—DC converter, more particularly, it relates to the prevention of a subharmonic oscillation occurring when an on-duty exceeds 50%.
2. Description of Related Art
FIG. 9 is a circuit diagram of a current mode DC—DC converter 100 of fixed off-time control. When a main switching transistor FET1 is turned on, an inductor current flowing through a choke coil L1 is increased. When the inductor current fed back via an input terminal FB1 becomes larger than an error amplification signal Vc, the transistor FET1 is turned off only for a fixed time. After the fixed off-time, the transistor FET1 is turned on again.
Moreover, Japanese unexamined patent publication Nos. 2002-223562 and 2005-143197 are disclosed as related art with the above.